Re: [Qemu-devel] [PATCH -V3 09/32] virtio-9p: Implement P9_TWRITE/ Thread model in QEMU

[jvrao]

Anthony Liguori wrote:
> On 03/29/2010 03:31 PM, Avi Kivity wrote:
>> On 03/29/2010 06:00 PM, Anthony Liguori wrote:
>>>
>>> In qemu, we tend to prefer state-machine based code.
>>>
>>> There are a few different models that we have discussed at different
>>> points:
>>>
>>> 1) state machines with a thread pool to make blocking functions
>>> asynchronous (what we have today)
>>>
>>> 2) co-operative threading
>>>
>>> 3) pre-emptive threading
>>>
>>> All three models are independent of making device models re-entrant. 
>>> In order to allow VCPU threads to run in qemu simultaneously, we need
>>> each device to carry a lock and for that lock to be acquired upon any
>>> of the public functions for the device model.
>>>
>>> For individual device models or host services, I think (3) is
>>> probably the worst model overall.  I personally think that (1) is
>>> better in the long run but ultimately would need an existence proof
>>> to compare against (2).  (2) looks appealing until you actually try
>>> to have the device handle multiple requests at a time.
>>
>> Sooner or later nature and the ever more complicated code will force
>> us towards (3).  As an example, we've observed live migration to
>> throttle vcpus when sending a large guest's zeroed memory over; the
>> bandwidth control doesn't kick in since zero pages are compressed, so
>> the iothread spends large amounts of time reading memory.
> 
> Making things re-entrant is different than (3) in my mind.
> 
> There's no reason that VCPU threads should run in lock-step with live
> migration during the live phase.  Making device models re-entrant and
> making live migration depend not depend on the big global lock is a good
> thing to do.
> 
> What I'm skeptical of, is whether converting virtio-9p or qcow2 to
> handle each request in a separate thread is really going to improve
> things.  The VNC server is another area that I think multithreading
> would be a bad idea.

Excuse me for some basic questions..still trying to understand QEMU concepts..

How does IO thread is accounted for? 

If I start a 2 CPU QEMU, we will be occupying two physical CPUs with two VCPU 
threads
and IO thread runs on other CPUs owned by the host right? If the answer is yes, 
then
we are good as the fileserver load has to be on the host CPUs. 

> 
>> We could fix this by yielding every so often (and a patch will be
>> posted soon), but it remains an issue.  We have too much work in the
>> I/O thread and that defers I/O completion and timers.
>>
>>> For virtio-9p, I don't think (1) is much of a burden to be honest. 
>>> In particular, when the 9p2000.L dialect is used, there should be a
>>> 1-1 correlation between protocol operations and system calls which
>>> means that for the most part, there's really no advantage to
>>> threading from a complexity point of view.
>>
>> But if those system calls are blocking, you need a thread?
> 
> You can dispatch just the system call to a thread pool.  The advantage
> of doing that is that you don't need to worry about locking since the
> system calls are not (usually) handling shared state.

How is locking avoided in the IO thread model? do we just have 1 IO thread 
irrespective of #VCPU threads? Is is that how the locking is avoided?

I think IO thread is used to do disk/network IO right? 
Putting 9P also on that will have any performance/scalability issues?

Thanks,
JV

> 
>> On a philosophical note, threads may be easier to model complex
>> hardware that includes a processor, for example our scsi card (and how
>> about using tcg as a jit to boost it :)
> 
> Yeah, it's hard to argue that script evaluation shouldn't be done in a
> thread.  But that doesn't prevent me from being very cautious about how
> and where we use threading :-)
> 
> Regards,
> 
> Anthony Liguori
>